Thermotolerance of the spinal cord after fractionated hyperthermia applied to the rat in the cervical region

The effects of heat treatment at 42.9 +/- 0.4 degrees C for 30-90 min of rat cervical spinal cord (cervical 5-thoracic 2) were investigated in animals which had received a priming treatment at 42.3 +/- 0.4 degrees for 60 min in the same region 24 hours before. Hyperthermia to the cervical region in the rat was applied by a coaxial double ring radiator, operating at a frequency of 434 MHz. The test heat treatment led to neurological complications, ranging from unco-ordinated use of the forelegs to paralysis of both forelegs. Death was observed, presumably as a result of respiratory paralysis. The animals that received the priming heat treatment were more resistant to heat injury than those without priming. The LD50, 60 days, for animals without and with priming treatment was respectively at 41 +/- 2 min 42.9 degrees C and at 88 +/- 7 min 42.9 degrees C. The TTR calculated from the LD50, 60 days, data is 2.1 +/- 0.2. Almost all animals, without priming, that did not die as a result of the treatment, recovered in the period from day 1 to day 60 after hyperthermia. Animals that had received a priming exposure were less able to recover from neurological symptoms induced by the test heat treatment. For this reason the induced resistance to thermal injury after priming exposure was more pronounced in the data on lethality than on neurological symptoms. Spinal cord histology from animals that did not recover from neurological symptoms showed extensive non-vascular damage to both gray and white matter.